The present invention relates to a semiconductor laser having a construction of multi-layers for lowering the forward voltage, and, more particularly, to a G.sub.a A.sub.s laser semiconductor enabled by molecular beam epitaxy method, and a method therefore.
Conventionally, what is shown in FIG. 8 is known as a semiconductor laser designated by reference numeral 21, which has a lower portion cladding layer 23 composed of N type Al.sub.x Ga.sub.1-x, As, an active layer 24 composed of Al.sub.x, G.sub.a1-x, As, a first upper portion cladding layer 25 composed of P type Al.sub.x G.sub.a1-x A.sub.s, a light absorption layer 26 composed of N type G.sub.a A.sub.s, and an evaporation preventing layer 27 composed of N type Al.sub.x, G.sub.a1-x, As are laminated on, for example, N type of GaAs type substrate 22. These respective layers 23 through 27 are formed by the use conventional apparatus in a known manner.
A stripe groove 29 is formed by means of photo etching from the evaporation preventing layer 27 to the light absorption layer 26. Furthermore, a second upper portion cladding layer 30 composed of P type Al.sub.y G.sub.a1-y A.sub.s and a cap layer 31 composed of P+ type G.sub.a A.sub.s are laminated to form as a regrowth enabled by means of conventional method. The first upper portion cladding layer 25 and the second upper portion cladding layer 30 come into contact against each other by the stripe groove 29. Electrodes 32a, 32b are formed respectively on the substrate 22 and the cap layer 31.
In the above-described conventional semiconductor laser 21, in the regrowth boundary A, i.e., the boundary between the first upper portion cladding layer 25 and the second upper portion cladding layer 30, a boundary level is formed. Thus, there is a problem that the series resistance component is increased and the forward voltage V.sub.F becomes higher.
Besides, as a method of manufacturing GaAs semiconductor lasers, for instance, the following processes i through iv are adapted in a known manner.
i:A first growth process, wherein a lower portion cladding layer, an active layer, a first upper portion cladding layer, a light absorption layer and an evaporation preventing layer are laminated on the surface of a G.sub.a A.sub.s substrate introduced into the growth chamber of a conventional apparatus for use in molecular beam epitaxy method. PA1 ii:A photo-etching process, wherein the laminated GaAs substrate is taken out of the growth chamber, and a stripe deep enough to reach down to the light absorption layer. PA1 iii:A reevaporation process, wherein the GaAs substrate with the stripe groove being formed is heated to selectively evaporate the light absorption layer. PA1 iv:A second growth process, wherein the second upper portion cladding layer and a cap layer are laminated on the GaAs substrate with the impurities being evaporated.
In the above-described conventional method for manufacturing G.sub.a A.sub.s semiconductor lasers, although the first upper portion cladding layer comes into contact with the second upper portion cladding layer in the stripe groove, there is a problem that the boundary order of the both is caused in the boundary, i.e., the current contraction portion and the forward voltage V.sub.F becomes higher.
It appeared that this problem can be settled by the increase in the carrier concentration of the first upper portion cladding layer. However, when the carrier concentration of the first upper portion cladding layer is increased, there is a problem that the other characteristics such as threshold current I th and so on are reduced.